Multi-link extensions and bundle skew management

ABSTRACT

A method, apparatus, and system are provided for multi-link extensions and bundle skew management. According to one embodiment, multiple parallel links between a central processing unit (CPU) and a peripheral device are combined into a single channel, and cells on the various links are received in a round-robin order, and variations in flight time between the various links are compensated through a timer at each receive port of the bundle.

RELATED APPLICATIONS

This is a Continuation Application of a prior U.S. application Ser. No.09/470,189, filed Dec. 22, 1999 now U.S. Pat. No. 6,646,991.

FIELD OF THE INVENTION

This invention relates to network server input/output (I/O) architectureand in particular, to aggregating bandwidth between a network server'scentral processing unit (CPU) and its I/O system by bundling multiplephysical links.

BACKGROUND OF THE INVENTION

The Next Generation Input/Output (NGIO) architecture, as described inNext Generation I/O Link Architecture Specification: Link Specificationpublished Mar. 26, 1999, is a channel-oriented, switched fabric, serialpoint-to-point link architecture aimed at meeting the growing needs ofI/O reliability, scalability and performance on servers. NGIO introducesthe use of an extremely efficient engine that is directly coupled tohost memory which replaces shared buses with a fabric of switchablepoint-to-point links. This approach decouples the CPU from the I/Osubsystem, as opposed to today's load/store memory-mapped I/O, andaddresses the problems of reliability, scalability, modular packaging,performance and complexity. CPU communication with peripherals occursasynchronously with the I/O channel engine being responsible for movingdata to and from main memory and allowing the bus to act as a switchwith point-to-point links capable of near linear scaling with CPU,memory and peripheral performance improvements. The use of standard,off-the-shelf components, such as the link physicals, also permit thisarchitecture to scale as higher bit rate frequencies become availableproviding backward compatibility and investment protection.

NGIO link architecture provides a method called Multiple Link Expansion(MLX) that aggregates the bandwidth of multiple parallel links toincrease bandwidth and reduce latency. MLX allows multiple links to beconnected between two devices. These parallel links can be bundledthrough MLX to work in concert as a single high bandwidth link.

During transmission, MLX transmits cells across bundled links. A bundleis made up of an ordered set of links. The transmitter must initiatecells across links of a bundle in a specified, round-robin order. Thereceiver knows this order and expects the cells to be distributed inthat order.

FIG. 1 is a block diagram of the prior art showing a normal transmissionin round-robin fashion over bundled links. Here, transmitter 100transmits three packets, A, B, and C. Each packet is segmented intoappropriately sized cells. Packet A consisting of two cells (A1 and A2),packet B consisting of three cells (B1, B2, and B3), and packet Cconsisting of a single cell (C1) are then sent from transmitter, 100,across bundled links connected to ports 104, 105, and 106. The order ofthe bundled links has been specified by a fabric manager as 104, 105 and106.

A cell is first dispatched from the port at the top of the round-robinorder, here 104. The next port in the round-robin order, 105, if stagedwith a cell, A2, may start transmitting that cell after the previousport, 104, has started to dispatch the previous cell, A1.

When the round-robin order reaches the last port in the order, 106, itwraps to the first port in the bundle, 104. To dispatch a cell, eachport must wait until the previous port starts transmitting the previouscell. A port may start transmission at the same time as the previousport but must not transmit any earlier. FIG. 2 is a block diagram of theprior art showing such a simultaneous transmission. Transmission doesn'toccur until the port satisfies the minimum inter-cell gap requirement.

All ports of the bundle must follow this MLX order behavior. If the nextport in the round-robin order does not have a cell staged, theround-robin ordering does not advance.

The receiver expects cells to arrive in a specified, round-robin order.FIG. 3 is a block diagram of the prior art showing a typical receptionin round-robin fashion over bundled links. Here, packet A consisting offour cells, A1-A4, is sent to receiver 113 across a four-link bundleconsisting of links 108, 109, 110 and 111. The reception sequence in theexample is port 114,115,116, then 117. Port 114 is currently at the topof the receive round-robin order. All sequence numbers were correctlyapplied by the transmitter. Each cell follows MLX transmission orderfrom the perspective of the transmitter.

Under MLX, cells must be received in the expected round-robin order.Links bundled together under MLX must use the same transmission rate.However, even links with the same transmission rate may have differentflight times due to physical characteristics such as length. Therefore,cells may sometimes be received in violation of the orderingrequirement.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth the features of the invention withparticularity. The invention, together with its advantages, may be bestunderstood from the following detailed description taken in conjunctionwith the accompanying drawings of which:

FIG. 1 is a block diagram of the prior art showing a normal staggeredtransmission in round-robin fashion over bundled links;

FIG. 2 is a block diagram of the prior art showing a normal simultaneoustransmission in round-robin fashion over bundled links;

FIG. 3 is a block diagram of the prior art showing a typical receptionin round-robin fashion over bundled links;

FIG. 4 is a block diagram of one embodiment of the present invention;

FIG. 5 is a block diagram of a reception in round-robin fashion overbundled links in which the reception has been skewed but remains validaccording to one embodiment of the present invention;

FIG. 6 is a block diagram of a reception in round-robin fashion overbundled links in which the reception has been skewed but remains validaccording to one embodiment of the present invention;

FIG. 7 is a flow diagram of bundle skew management according to oneembodiment of the present invention;

FIG. 8 is a block diagram of a reception in round-robin fashion overbundled links in which the reception has been excessively skewedresulting in a failure due to a dropped cell according to one embodimentof the present invention;

FIG. 9 is a flow diagram of cell synchronization according to oneembodiment of the present invention; and

FIG. 10 is a flow diagram of link failure processing according to oneembodiment of the present invention.

DETAILED DESCRIPTION

A method and apparatus are described for aggregating bandwidth ofmultiple parallel links between two devices to increase bandwidth andreduce latency. These parallel links can be bundled to work in concertas a single high bandwidth link. The various links of the bundle arehandled in a round-robin order. Variations in flight time between thevarious links are compensated for through a timer at each receive portof the bundle.

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout some of these specific details. In other instances, well-knownstructures and devices are shown in block diagram form.

The present invention includes various steps, which will be describedbelow. The steps of the present invention may be performed by hardwarecomponents or may be embodied in machine-executable instructions, whichmay be used to cause a general-purpose or special-purpose processor orlogic circuits programmed with the instructions to perform the steps.Alternatively, the steps may be performed by a combination of hardwareand software.

Importantly, while embodiments of the present invention will bedescribed with reference to NGIO and bundled physical links between anetwork server's CPU and its I/O system, the method and apparatusdescribed herein are equally applicable to other systems using multiplephysical links between devices.

One embodiment of the present invention is illustrated in FIG. 4. Here,a transmitter 100 and a receiver 113 are shown for ease of explanation.However, it should be realized that transmissions between these twodevices can be duplex and therefore each device can act as both atransmitter and receiver as necessary. The transmitter 100 can be adevice such as a host channel adapter (HCA) and the receiver 113 can bea device such as a target channel adapter (TCA).

The transmitter 100 and the receiver 113 can be connected by severalphysical links 108-111. These links are connected to the transmitter atports 104-107 and to the receiver at ports 114-117. Links 108-111 arebundled together to form one logical channel 112. Packets to betransmitted across channel 112 are separated into appropriately sizedcells by the segmentation controller 102. These cells are thentransmitted from ports 104-107 in an appropriate round-robin orderdetermined by the round-robin controller 103.

Cells arriving at the receiver ports 114-117 are temporarily stored inthe incoming message buffers 118. The cells are read from the ports inan appropriate round-robin order determined by the round-robincontroller 119. These cells are then recombined into the originalpackets by the reassembly controller 121.

Links in a bundle may have different flight times. Therefore, cellswhich are transmitted following MLX ordering behavior may arrive at thereceiver in apparent violation of this ordering behavior. In oneembodiment of the current invention this skew is compensated for througha timer at each receive port of the bundle. The timer is loaded withtime T_(skew). MLX does not specify the timeout which can be long andconservative since it is not the primary mechanism for detecting lostcells or validating good cells. A longer timeout allows cables ofsignificantly different flight time characteristics, such as differentcable lengths, to be successfully bundled. A longer timeout requiresdeeper buffers and produces some additional latency in servicing goodcells in other receive port buffers, but cells will not be lost ormistaken. Preferably, the T_(skew) value represents the worst-caseflight time of the smallest possible cell (e.g., 20 bytes) across theparticular physical media.

Bundle Skew Management Processing

In one embodiment of the current invention when a cell arrives on anunexpected link of a bundle, the receiver continues waiting for a cellto arrive on the expected link for a time interval equal to Tskew. Thisis called the bundled link skew value.

In one embodiment of the current invention illustrated in FIG. 5, packetA is a packet consisting of four cells (A1-A4) which is sent across fourbundled links 108-111. These links are connected to receiver ports114-117. The round-robin order of the bundled links is receiver port114, 115, 116 then 117. Port 114 is currently at the top of theround-robin order. The transmitter correctly assigns all sequencenumbers. Each cell follows the MLX transmission order from theperspective of the transmitter.

Unexpected cell arrival can occur on any link, or any number of links.According to embodiments of the present invention, if the arrival occursbefore the expiration of the tskew timer for the expected link, thereception is still valid. FIG. 6 is a block diagram of a reception inround-robin fashion over bundled links in which the reception has beenskewed but remains valid according to on embodiment of the presentinvention. Here, port 114 is at the top of the round-robin order. CellsA4, A3, and A2 all arrive before A1 but the reception is still validbecause Al arrived before the expiration of the tskew timer, 120. Atskew failure occurs when the expected receive port tskew window isexceeded. This failure can only occur in a link which drops a cell. Inone embodiment, under conditions of tskew failure, the receiver advancesthe expected link, round-robin order, to the next link which has alreadyreceived a cell.

This process, according to one embodiment of the present invention, isfurther described in FIG. 7. First, a cell is received 701. Next, adetermination is made as to whether this cell is out of the proper order702. This determination is based on whether the cell arrived on the nextexpected link of the round-robin order. If the cell is out of order,that is, arrived on a link other than the expected link, the receiverwill wait for a period up to the value of the tskew timer for theexpected link 7O3. If a cell has not yet arrived on the expected link7O4, that link is removed from the round-robin order 705 and the cellsynchronization process can be done.

Cell Synchronization Process

FIG. 8 is a block diagram of a reception in round-robin fashion overbundled links in which the reception has been excessively skewedresulting in a failure due to a dropped cell according to one embodimentof the present invention. Here, port 114 is currently at the top of thereceiver round-robin order. The receiver expects a cell to arrive atport 114. When the cell (A2) arrives at port 115, the receiver continueswaiting for a cell at port 115 for an additional tskew time window. Whenthe expected cell on port 114 does not arrive, the receiver advancesport 115 to the top of the round-robin order, overriding and resettingsequence number and round-robin expectations.

In one embodiment of the current invention illustrated in FIG. 9, thisprocess is entered when more than a simple cell skip is detected.Basically, detecting skipped cells means that the expected cell doesn'tmatch the bundle sequence number (BSN) 901 and the next expected celldoesn't match BSN+1 902. Current cells in the other port's buffersshould be emptied out in a best effort fashion. In this mode, the lowestBSN cell at the head of a port buffer is always processed first 903-904.When this technique establishes a linear sequence, N times in a row(N=number of links in bundle) 905-906 then both transmitter and receiverhave re-established synchronization and can go into normal servicingmode. MLX does not specify how to determine the lowest BSN cell. MLX canbe a fast parallel process, or a slow serial, scan process. The receiveris expected to use flow control to throttle the transmitter if thisprocess takes a long time.

Transmitter Behavior on Failed Link of Bundle

In one embodiment of the current invention illustrated in FIG. 10, whena transmitter detects that a link of the bundle has failed (link losessync), the receiver sends a trap to a fabric manager 1001 and removesthe failed link from the round robin order. The receiver then continuesto handle the other links in the round-robin order 1002. If the roundrobin order reaches the failed link 1003 and the link has notresynchronized 1004 the receiver skips the BSN corresponding to thefailed link and continues to exclude that link from the round-robinorder 1005. Until the node receives a new bundle configuration or thefailed link resynchronizes by itself, the transmitter dispatches allcells, in order, over the remaining functional links.

This dispatch differs from normal operation in one aspect. When thetransmitter passes the position in the round-robin order held by afailed link, the transmitter skips a BSN corresponding to the skippedposition previously held by that link.

This behavior guarantees minimal data loss caused by a link failure, andposes minimal disruption to the receive cell synchronization process.

Flow Control

MLX does not affect link level flow control. If a cell arrives on anunexpected link of a bundle, depending on the offset of the links fromexpected to unexpected and the depth of its receive buffers, thereceiver may choose to flow control the link until the portsynchronization event is resolved.

Flow control on a link of a bundle may affect other links of the bundle.Assertion by the receiver of flow control on a link of the bundle willaffect the transmitted flow behavior on other links of the bundle whenthe flow-restricted link is at the top of the round-robin order. As longas the previous conditions are met, all bundled links will behaveaccording to the flow control of the most restrictive link.

In view of the many possible embodiments to which the principles of ourinvention may be applied, it will be appreciated that the embodimentsand aspects thereof described herein with respect to thedrawings/figures are only illustrative and should not be taken aslimiting the scope of the invention. To the contrary, the invention asdescribed herein contemplates all such embodiments as may come withinthe scope of the following claims and equivalents thereof.

1. A method, comprising compensating for variations in flight timebetween a plurality of links through a timer at each receive port of abundle by determining whether a cell of a plurality of cells is receivedon an unexpected link of the plurality of links, receiving the cell onthe unexpected link, a receiver waiting for the cell to arrive on theexpected link for a time period, the time period including a value ofthe timer of the expected link, wherein the value of the timerrepresents the worst-case flight time of the smallest possible cell ofthe plurality of cells across a particular physical link of theplurality of links; advancing the expected link to the next link thathas already received a cell when the previous expected link has notreceived a cell within the value of the timer; and correcting cellsynchronization by emptying port buffers in a best effort fashion; andprocessing the cell with the lowest bundle sequence number first.
 2. Themethod of claim 1, further comprising determining synchronization of alink based on the occurrence of a linear sequence of bundle sequencenumbers a number of times in a row.
 3. The method of claim 2, whereinthe number equals the number of links in a bundle.
 4. The method ofclaim 1, further comprising: removing a failed link of the plurality oflinks from the round-robin order; sending a trap to a fabric manager;and dispatching the plurality of cells over the remaining links of theplurality of links until the failed link receives a new bundleconfiguration or resynchronizes.
 5. The method of claim 4, furthercomprising skipping a bundle sequence number corresponding to the failedlink when a transmitter passes the failed link in the round-robin order.6. An apparatus, comprising: a central processing unit (CPU); and areceiver coupled with the CPU, the receiver to receive a plurality ofcells using a plurality of links of a bundle in a round-robin order,compensate for variations in flight time between the plurality of linksthrough a timer at each receive port of the bundle by determiningwhether a cell of the plurality of cells is received on an unexpectedlink of the plurality of links and, receive the cell on the unexpectedlink, wait for the cell to arrive on an expected link for a time period,the time period including a value of the timer of the expected link,wherein the value of the timer represents the worst-case flight time ofthe smallest possible cell of the plurality of cells across a particularphysical link of the plurality of links, remove a failed link from theround-robin order, send a trap to a fabric manager, and dispatch theplurality of cells over the remaining links of the plurality of linksuntil the failed link receives a new bundle configuration orresynchronizes.
 7. The apparatus of claim 6, wherein the receiver isfurther to advance the expected link to the next link that has alreadyreceived a cell when the previous expected link has not received a cellwithin the value of the timer.
 8. The apparatus of claim 7, wherein thereceiver is further to correct cell synchronization by: emptying portbuffers in a best effort fashion; and processing the cell with thelowest bundle sequence number first.
 9. The apparatus of claim 8,wherein the receiver is further to determine synchronization of a linkbased on the occurrence of a linear sequence of bundle sequence numbersa number of times in a row.
 10. The apparatus of claim 9, wherein thenumber equals the number of links in the bundle.
 11. The apparatus ofclaim 6, wherein the receiver is further to skip a bundle sequencenumber corresponding to the failed link when the transmitter passes thefailed link in the round-robin order.
 12. A computer-readable mediumencoded with a data structure comprising instructions which, whenexecuted, cause the machine to compensate for variations in flight timebetween a plurality of links through a timer at each receive port of abundle by determining whether a cell of a plurality of cells is receivedon an unexpected link of the plurality of links and, receive the cell onthe unexpected link, wait for the cell to arrive on an expected link forup to a time period, the time period including a value of the timer ofthe expected link, wherein the value of the timer represents theworst-case flight time of the smallest possible cell of the plurality ofcells across a particular physical link of the plurality of links,remove a failed link from the round-robin order, send a trap to a fabricmanager, and dispatch the plurality of cells over the remaining links ofthe plurality of links until the failed link receives a new bundleconfiguration or resynchronizes.
 13. The computer-readable medium ofclaim 12, wherein the instructions which, when executed, further causethe machine to advance the expected link the next link that has alreadyreceived a cell when the previous expected link has not received a cellwithin the value of the timer.
 14. The computer-readable medium of claim12, wherein the instructions which, when executed, further cause themachine to skip a bundle sequence number corresponding to the failedlink when a transmitter passes the failed link in the round-robin order.15. A system, comprising: a storage medium; a central processing unit(CPU) coupled with the storage medium; and a receiver coupled with theCPU, the receiver to receive a plurality of cells using a plurality oflinks of a bundle in a round-robin order, compensate for variations inflight time between the plurality of links through a timer at eachreceive port of the bundle by determining whether a cell of theplurality of cells is received on an unexpected link of the plurality oflinks and, receive the cell on the unexpected link, wait for the cell toarrive on an expected link for a time period, the time period includinga value of the timer of the expected link, wherein the value of thetimer represents the worst-case flight time of the smallest possiblecell of the plurality of cells across a particular physical link of theplurality of links, remove a failed link from the round-robin order,send a trap to a fabric manager, and dispatch the plurality of cellsover the remaining links of the plurality of links until the failed linkreceives a new bundle configuration or resynchronizes.
 16. The system ofclaim 15, wherein the receiver is further to advance the expected linkto the next link that has already received a cell when the previousexpected link has not received a cell within the value of the timer. 17.The system of claim 15, wherein the receiver is further to skip a bundlesequence number corresponding to the failed link when the transmitterpasses the failed link in the round-robin order.